1. Field of the Invention
This invention relates to electron beam resists for photomask fabrication and for semiconductor device fabrication.
2. Description of the Prior Art
The use of light as the irradiator for fabricating photoresists in the semiconductor art has been common for many years. The photoresist method of semiconductor manufacture was adequate until the advent of small geometry high frequency devices and integrated circuits requiring the formation of patterns with line widths in the neighborhood of one micron. Although one micron line opening, or resolution, can be obtained from photoreists in the laboratory, such line widths are not reproducible in production due to diffraction problems, with the practical limit of production produced openings being in the neighborhood of five to six microns in width.
The step from the use of light to electrons to form resists was a logical one. In the case of electron beam technology, an electron beam is scanned across the resist itself to form the desired pattern. The electron beam is controlled by a computer which has been fed the coordinates of the pattern as previously determined by a designer. Thus, the use of the electron beam has eliminated all the time lost in preparing the reduction photography required to form a patterned photoresist. However, due to the pattern in the electron beam resist resulting from the scan of a very narrow electron beam, the reaction time of the resist to the electron beam is the time drawback to the production use of electron beam resists.
Obviously, then, in addition to the characteristics required of a good photoresist, such as: good adhesion to many materials, good etch resistance to conventional etches, solubility in desired solvents, and thermostability, an electron resist must react to the electron beam irradiation fast enough to allow a reasonable scan time of the electron beam. In addition, in order to bring electron beam technology into production status, resists composed of thin polymer films that are capable of retaining an image of one micron or less at very high scanning speeds of the electron beam are required.
A number of approaches have been taken in the past to develop practical electron beam resists. The first approach and one that proved to be the least successful was the use of conventional photoresists, which are also polymers. Although capable of being exposed at relative high scan rates, such resists exhibit line widths, i.e., resolutions, greater than one micron in width.
The most widely used electron beam resist today is polymethyl methacrylate (PMMA), a positive resist; a positive resist being defined as a polymer that is insoluble in certain solvents but will degrade and become soluble when electron beam irradiates, a negative resist being a polymer that is soluble in certain solvents until irradiated. PMMA is characterized by excellent resolution and line width characteristics and by good processability. However, PMMA requires a relatively slow exposure rate, of approximately 5 .times. 10.sup.-5 coulombs/cm.sup.2, and has the inability to withstand strong oxidizing acids and base etches.
Theoretically, since the size of an electron is only about 1/1000th the size of a quantum of light, an electron beam should produce openings with line widths several orders of magnitude smaller than the size of openings obtained with photoresists. However, due to the electron back-scatter from the surface supporting the resist, such small width openings have not been obtained in the past, 0.7 microns being the practical lower limit in size. Because the back-scattering causes either degradation of the back-scattered area in the case of positive resists or causes cross linkage in the back-scattered area in the case of negative resists, the sides of the patterned openings in the resist, instead of being perpendicular to the support surface, tend to slope to a knife edge. The electrons bounce back through the resist at various angles between 90.degree. and 180.degree., thus exposing the resist over a much larger cross sectional area than the diameter of the electron beam. An obvious solution to this problem of electron back-scatter is to use thinner films since the distance subtended by an angle increases with the distance from the origin. However, in most cases, this is not a practical solution because the resist must not be effected by the harsh etchants which are used to etch openings in the support; the thicker the resist, the greater the resistance of the resist to destruction by the etches. In addition, a thicker film is desirable for it will tend to cover any dust or other impurity particles on the support or in the resist itself, and will tend to cover pinholes in the resist thereby furnishing a much more uniform film.
Therefore, an object of this invention is to provide a method of forming an electron beam resist with better resolution and narrower width openings than is presently possible.
Another object of this invention is to provide a method of forming an electron beam resist that has all of the characteristics required of good electron beam resists, such as fast scanning speed, good adhesion to many materials, good processability and thermostability.
Another object of this invention is to provide a method of forming an electron beam resist that has openings with sides perpendicular to the support.
A further object of this invention is an electron beam resist having better resolution and narrower width openings than presently possible with current resists.
Yet another object of this invention is to provide an electron beam resist that has openings with sides perpendicular to the support.
Another object of this invention is to provide an electron beam resist that has all of the required characteristics required of good electron beam resists, such as fast scanning speed, good adhesion to many materials, good processability and thermostability.